The present invention generally relates to a device for producing a blade having two outer walls and at least one cavity, arranged between the outer walls, for a turbine, comprising an outer mold and a plurality of cores for forming the outer walls and the at least one cavity.
The invention also generally relates to a method of producing a blade having two outer walls and at least one cavity, arranged between the outer walls, for a turbine, an outer mold and a plurality of cores being provided for forming the outer walls and the at least one cavity.
Further general subject matter of the invention includes a blade for a turbine, in particular a gas turbine, having two outer walls and at least one cavity arranged between the outer walls.
Blades, in particular blades for gas turbines, must be cooled from inside on account of the high operating temperatures. For this purpose, the blades have one or more cavities. In the hitherto known blades, these cavities extend from the one outer wall of the blade up to the other outer wall. A section of a core is provided for forming each cavity. The individual sections are connected to one another. The core is accommodated in a suitable receptacle of an outer mold for producing the blade by a casting process. In this case, the length of the core can assume comparatively high values.
In blades cooled from inside, the wall thickness of the outer walls is to be selected to be as small as possible. A substantial improvement in the cooling can be achieved by a small wall thickness. The minimum wall thickness provided must always be greater than the tolerance of the wall thickness. Otherwise, there is the risk of the core being displaced and/or deformed during the casting in such a way that it comes into contact with the outer mold and the blade produced has a hole. In practice, therefore, a comparatively large wall thickness must be selected.
A further disadvantage of the known methods is that shifting of the core during the casting has consequences for both outer walls of the blade. The reason for this is that the core extends from the one outer wall up to the other outer wall. Therefore, the core has to be produced with high precision in these known methods. Tolerances which occur during the production of the core must likewise be taken into account.
To improve the cooling, blades having cavities are known. Such a blade and also a method and a device for producing it have been disclosed by WO 99/59748 originating from the same applicant. This publication proposes a multiplicity of cores which are connected to one another and the outer mold via connecting elements. The production of this blade is complicated and costly.
An object of a preferred embodiment of the present invention is therefore to provide a simple and cost-effective device and a cost-effective method for producing a blade with small wall thicknesses. A further object of an embodiment of the invention is to provide a blade for a turbine, this blade having outer walls with a substantially smaller wall thickness.
The device according to an embodiment of the invention provides for each of the cores to have at least one section which extends from an associated outer wall up to a center web of the blade without being involved in the formation of the other outer wall.
The method according to an embodiment of the invention is characterized in that at least one section of each core is supported in such a way that the distance between the outside of the section of the one core and the inside of the outer mold is independent of the distance between the outside of the section of the other core and the inside of the outer mold, so that the wall thicknesses of the two outer walls, at least in the region of the sections, are formed independently of one another.
According to an embodiment of the invention, in a blade of the type mentioned at the beginning, this object is achieved in that at least one cavity is divided into two passages by a center web, the one passage being arranged between the one outer wall and the center web and the other passage being arranged between the center web and the other outer wall.
One basic idea of an embodiment of the invention is that the two outer walls of the blade are produced independently of one another at least in sections. At least one cavity of the blade is divided into two passages by a center web. The one passage extends from the first outer wall up to the center web and the other passage extends from the center web up to the second outer wall. A plurality of cores are provided. A first core has one or more sections for forming the passages between the first outer wall and the center web. The further passages are formed by sections of a second core which is provided separately from the first core. Displacements and deformations of the first core which bring about a change in the wall thickness of the one outer wall are not transmitted to the second core. The wall thickness of the two outer walls, at least in regions, are therefore formed independently of one another.
The method according to an embodiment of the invention provides for those sections of each core which serve to form the passages to be supported in such a way that a minimum wall thickness is ensured. Projections which are supported on the inside of the outer mold are advantageously used for this purpose.
During the production of the cores, only the outside, facing the inside of the outer mold, of the sections is critical for the wall thickness of the outer walls. In particular, comparatively coarse tolerances may be applied to the side of the sections which is assigned to the center web. As a result, the production accuracy of the outside of the cores can be substantially improved, the outside of the cores being critical for the wall thickness of the outer walls. All the tolerances are shifted into the region of the center web. This does not result in disadvantages for the cooling effect, since the hot fluid flowing through the turbine is not admitted directly to the center web. Furthermore, the center web is cooled on both sides by the passages. The center web also provides the requisite strength for the blade when the outer walls have small wall thicknesses.
According to an advantageous development of an embodiment of the invention, the cores are provided with projections for supporting on the outer mold. They are then advantageously supported on one another during the casting and pressed against the inside of the outer mold. The support may be effected by rigid, in particular wedge-shaped, or elastic spacers.
With this procedure, a minimum wall thickness for the outer walls is reliably maintained. Displacements of the cores toward the inside are avoided by the cores being supported on one another. For the production of the cores, this means that only the outside facing the inside of the outer mold has to be produced with high precision. Due to the two cores being supported on one another, the accuracy to size of the further outsides is only of secondary importance. Greater rigidity than in the known devices and methods is also achieved due to the cores being supported on one another. Displacements or deformations of the cores during the casting are therefore reduced. The tolerance range for the wall thickness of the outer walls can therefore be markedly reduced, so that thinner walls overall may be provided.
The projections serving for the support on the outer mold advantageously taper starting from the cores. In particular, they may be of conical design. This ensures that only point-like openings are produced in the outer walls, through which openings only minimum cooling medium escapes. Despite the support on the inside of the outer mold, the desired high cooling efficiency is therefore maintained.
The cores may be fixed at one or both ends in a receptacle of the outer mold in the longitudinal direction of the blade. Fixing solely in the longitudinal direction is sufficient if the support is effected in the transverse direction by the projections on the cores. The position of the cores during the production of the wax tool and during the casting is thereby ensured.
The outer walls are advantageously connected to one another via a plurality of ribs for forming a plurality of cavities. This results in specific cooling of individual regions of the blade with increased strength.
According to an advantageous development, a cavity at a leading edge and/or a trailing edge of the blade is free of the center web. The reason for this is that an increased cooling effect is required in the region of the leading edge. The cooling effect would be impaired in the junction region of the center web. This also correspondingly applies to the trailing edge.
In an advantageous configuration, the wall thickness of the center web is greater than the wall thickness of the outer walls. The requisite strength of the blade is then ensured by the center web and possibly the ribs. The wall thickness of the outer walls can accordingly be reduced.